docs: def add_filter(self, filter_values):
          """Improve the original one to deal with OR cases."""
          field = self._params['field']
          # Build a `AND` query on values wihtout the OR operator.
          # and a `OR` query for each value containing the OR operator.
          filters = [
              Q('bool', should=[
                  Q('term',  **{field: v}) for v in value.split(OR_SEPARATOR)
              ])
              if OR_SEPARATOR in value else
              Q('term',  **{field: value})
              for value in filter_values
          ]
          return Q('bool', must=filters) if len(filters) > 1 else filters[0]